Network Working Group T. Narten
Request for Comments: 2469 C. Burton
Category: Informational IBM
December 1998
A Caution On The Canonical Ordering Of Link-Layer Addresses
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
Abstract
Protocols such as ARP and Neighbor Discovery have data fields that
contain link-layer addresses. In order to interoperate properly, a
sender setting such a field must insure that the receiver extracts
those bits and interprets them correctly. In most cases, such fields
must be in "canonical form". Unfortunately, not all LAN adaptors are
consistent in their use of canonical form, and implementations may
need to explicitly bit swap individual bytes in order to obtain the
correct format. This document provides information to implementors
to help them avoid the pitfall of using non-canonical forms when
canonical forms are required.
Table of Contents
1. Introduction............................................. 22. Canonical Form........................................... 23. Implementors Beware: Potential Trouble Spots............. 33.1. Neighbor Discovery in IPv6.......................... 33.2. IPv4 and ARP........................................ 34. Security Considerations.................................. 35. References............................................... 46. Authors' Addresses....................................... 47. Full Copyright Statement................................. 5Narten & Burton Informational [Page 1]

RFC 2469 Canonical Ordering Of Link-Layer Addresses December 19981. Introduction
Protocols such as ARP [ARP] and ND [DISCOVERY] have data fields that
contain link-layer addresses. In order to interoperate properly, a
sender setting such a field must insure that the receiver extracts
those bits and interprets them correctly. In most cases, such fields
must be in "canonical form". Unfortunately, not all LAN adaptors are
consistent in their use of canonical form, and implementations may
need to explicitly bit swap individual bytes in order to obtain the
correct format.
2. Canonical Form
Canonical form (also known as "LSB format" and "Ethernet format") is
the name given to the format of a LAN adapter address as it should be
presented to the user according to the 802 LAN standard. It is best
defined as how the bit order of an adapter address on the LAN media
maps to the bit order of an adapter address in memory: The first bit
of each byte that appears on the LAN maps to the least significant
(i.e., right-most) bit of each byte in memory (the figure below
illustrates this). This puts the group address indicator (i.e., the
bit that defines whether an address is unicast or multicast) in the
least significant bit of the first byte. Ethernet and 802.3 hardware
behave consistently with this definition.
Unfortunately, Token Ring (and some FDDI) hardware does not behave
consistently with this definition; it maps the first bit of each byte
of the adapter address to the most significant (i.e., left-most) bit
of each byte in memory, which puts the group address indicator in the
most significant bit of the first byte. This mapping is variously
called "MSB format", "IBM format", "Token-Ring format", and "non-
canonical form". The figure below illustrates the difference between
canonical and non-canonical form using the canonical form address
12-34-56-78-9A-BC as an example:
In memory, 12 34 56 78 9A BC
canonical: 00010010 00110100 01010110 01111000 10011010 10111100
1st bit appearing on LAN (group address indicator)
|
On LAN: 01001000 00101100 01101010 00011110 01011001 00111101
In memory,
MSB format: 01001000 00101100 01101010 00011110 01011001 00111101
48 2C 6A 1E 59 3D
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RFC 2469 Canonical Ordering Of Link-Layer Addresses December 1998
The implication of this inconsistency is that addresses extracted
from adaptors, assigned to adaptors, or extracted from link-layer
packet headers obtained from adaptors may need to be bit-swapped to
put them into canonical form. Likewise, addresses in canonical form
that are handed to adaptors (e.g., to set an address, to specify a
destination address in a link-layer header, etc.) may need to be
bit-swapped in order for the adaptor to process the request as
expected.
3. Implementors Beware: Potential Trouble Spots3.1. Neighbor Discovery in IPv6
All of the IPv6 over specific link layers documents specify that
link-layer addresses must be transmitted in canonical order [IPv6-
ETHER, IPv6-FDDI, IPv6-TOKEN]. As far as the authors can tell, all
Ethernet LAN adaptors use canonical order and no special processing
by implementations is needed. In contrast, some FDDI and all Token
Ring adaptors appear to use non-canonical format. Implementors must
insure that any addresses that appear in link-layer address options
of Neighbor Discovery [DISCOVERY] messages are sent in canonical
order and that any link-layer addresses extracted from ND packets are
interpreted correctly on the local machine and its adaptors.
3.2. IPv4 and ARP
Ethernet addresses that appear in ARP packets are in canonical order.
In contrast, when running ARP over Token Ring, the de facto practice
is to transmit addresses in non-canonical order. Because all Token
Ring adaptors assume non-canonical ordering, no interoperability
problems result between communicating nodes attached to the same
Token Ring.
In some environments, however, Token Rings and Ethernets are
connected via a bridge. When a node on the Token Ring attempts to
communicate with a node on the Ethernet, communication would normally
fail, since the Ethernet will misinterpret the Token Ring address
(and vice versa). To get around this problem, bridges that forward
packets between dissimilar network types perform bit swaps of the
addresses in the address fields of ARP packets that are forwarded
from a network of one type to one of the other.
4. Security Considerations
There are no known security issues raised by this document.
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RFC 2469 Canonical Ordering Of Link-Layer Addresses December 19987. Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights Reserved.
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